Diverging Pathways in the Activation of Allenes with Lewis Acids and Bases: Addition, 1,2-Carboboration, and Cyclization † ‡ † † ‡ ‡ Rebecca L

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Diverging Pathways in the Activation of Allenes with Lewis Acids and Bases: Addition, 1,2-Carboboration, and Cyclization † ‡ † † ‡ ‡ Rebecca L. Melen,*, , Lewis C. Wilkins, Benson M. Kariuki, Hubert Wadepohl, Lutz H. Gade, § ∥ ⊥ § A. Stephen K. Hashmi, , Douglas W. Stephan, and Max M. Hansmann*, † School of Chemistry, Cardiﬀ University, Main Building, Cardiﬀ CF10 3AT, Cymru/Wales, U.K. ‡ § Anorganisch-Chemisches Institut and Organisch-Chemisches Institut, Ruprecht-Karls-Universitaẗ Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany ∥ Chemistry Department, King Abdulaziz University (KAU), Jeddah 21589, Saudi Arabia ⊥ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6

*S Supporting Information

ABSTRACT: The reactions of allenes with frustrated (or cooperative) Lewis acid/base pairs result in the 1,4-addition of the base pair to the allene. The reactions of allenyl ketones and esters just in the presence of the strong Lewis acid B(C6F5)3 aﬀord the selective formation of the 1,2-carboboration products. In both cases the Lewis acid activates the allene to either a C6F5 migration or nucleophilic attack by the Lewis base. In addition to the 1,2-carboboration pathway, which can be viewed as being triggered by activation of the ketone (σ- activation), in the case of allenyl esters the corresponding cyclization products are observed in the presence of water.

■ INTRODUCTION Scheme 1. Boron-Mediated π Activation of Alkenes and Alkynes The chemistry of frustrated Lewis pairs (FLPs) has exploded since their ﬁrst report in 2006.1 The combination of a Lewis acid and a Lewis base where steric hindrance impedes classical Lewis acid/base adduct formation constitutes an FLP. Owing to the unquenched Lewis acidity and basicity, such systems display unique reactivity and are capable of activating small molecules such as H2,CO2,N2O, SO2, alkenes, and alkynes, 2 inter alia. In the case of H2 activation, FLPs have been employed in metal-free hydrogenation catalysis for the reduction of a variety of unsaturated substrates,2,3 including enantioselective hydrogenations.4 FLPs have also been shown to eﬀect 1,2-additions to both alkynes and alkenes. In these cases, the Lewis base (e.g., amines, phosphines, thiols, or amides) acts as the nucleophile and the Lewis acid (usually a bond complex has never been isolated, although weak van der borane or alane) as the electrophile (Scheme 1, path I).5 This Waals interactions have been detected.11 In FLP chemistry, has also been extended to 1,4-addition processes in the case of computational studies have shown that an “encounter complex” 6 7 1,3-dienes, 1,3-enynes, and 1,3-diynes. In this context, we forms between the Lewis acid (B(C6F5)3), the Lewis base have implemented the strong Lewis acid B(C6F5)3 to activate (tBu3P), and ethylene in which both the Lewis acid and base the alkyne moiety, thus promoting intramolecular attack by components of the FLP interact simultaneously with the 12 nucleophilic amide or ester functionalities.8,9 In the case of alkene. In these calculations, the addition reaction was shown − terminal propargyl esters intramolecular cyclization onto the π- to be asynchronous, with the C B bond forming slightly before P−C bond formation. However, in other cases it was calculated activated alkyne followed by a subsequent C6F5 shift from that the borane initially associated with ethylene, which was boron to carbon (1,1-carboboration) leads to cyclic allyl boron 13 reagents, which can then undergo a rearrangement reaction to then followed by 1,2-FLP addition. yield the formal 1,3-carboboration products (Scheme 1, path II).9 While π-Lewis acidic transition metals such as gold can Received: June 23, 2015 interact strongly with π-bonds,10 the corresponding boron−π- Published: August 13, 2015

Carboboration reactions of alkynes result in the breaking of Interestingly, Alcarazo et al. studied the FLP-catalyzed − fi the C B bond in the R3B starting material and the formation of hydrogenation of electron-de cient allenes, although the − − 14,15 both R CandCBR2 bonds providing a versatile addition reactions of FLPs in the absence of hydrogen were synthetic method to substituted alkenylboranes. Such species not described.22 have found applications in a variety of organic transformations, including cross-coupling reactions.16 Typically, transition- ■ RESULTS AND DISCUSSION metal-based catalysts such as those with copper, palladium, Reactivity of Allenyl Ketones toward Cooperative and nickel have been utilized to effect carboboration Lewis Pairs. We initially investigated the reactions of Lewis 17 reactions, although Ohmiya and Sawamura et al. recently acid/base pairs with allenes in the anticipation that the Lewis reported an elegant approach to afford 1,2-trans-carboboration acid and base could add across the terminal alkene unit of the products using phosphine organocatalysis with alkynoates allene similarly to that observed for the 1,2-addition of FLPs 18 (Scheme 2, path I). Earlier reports of metal-free, noncatalyzed with alkenes and alkynes (Scheme 1, path I).5 The allene starting materials 1a−e bearing an electron-withdrawing ketone Scheme 2. Catalyzed and Noncatalyzed Carboboration functionality were synthesized according to standard literature Reactions protocols, involving homopropargylation mediated by the in situ generated organozinc compounds followed by Dess− Martin periodinane oxidation (see the Supporting Informa- tion). The allenyl ketones 1a−e were subsequently reacted with the Lewis acid B(C6F3)3 and a tertiary phosphine (P(o-tol)3, P(m-FC6H4)3,P(p-OMeC6H4)3, PBn3) in a 1:1:1 stoichiometric ratio. The in situ NMR-scale reaction of 1a−e with the Lewis acid/base combination in CDCl3 resulted in an immediate color change to give orange or red solutions in all cases. The reactions were extremely rapid and clean, as observed by NMR (1H, 19F, 11B, 13C, 31P) spectroscopy, yielding the products 2a−k (Scheme 3) quantitatively by NMR spectroscopy under ambient conditions, although the isolated yields were sometimes lower. The 11B NMR spectra were all very similar for compounds 2, displaying a resonance between

Scheme 3. Reactions of Allenyl Ketones 1 with FLPs To Give a 2

carboboration reactions described as the “Wrackmeyer reaction” involved “activated” alkynes such as acetylides derived from the heavier group 14 elements or terminal metal alkynyl species which reacted in the presence of trialkylboranes.14,19 In these cases, a 1,2-shift reaction leads to 1,1-carboboration products which are postulated to proceed through the formation of alkynylborates (Scheme 2, path II). In the past few years, the Erker group extended this methodology to “normal” terminal and internal alkynes utilizing the strong Lewis acids B(C6F5)3 and RB(C6F5)2 to generate alkenylboranes in a 1,1-carboboration reaction (Scheme 2, path III)14,15 in contrast to 1,2-hydroborations.20 However, recent work by Ingleson and Bourissou has described borenium systems that eﬀect metal-free syn-1,2-carboborations: for example, in the selective transfer of phenyl and thienyl groups from the borenium system to the alkyne (Scheme 2, path IV).21 Herein, we report the reactions of (frustrated) Lewis acid/ base pairs with allenes and also show that in the absence of the Lewis base metal-free regioselective 1,2-carboboration reactions a of allenes take place using the strong Lewis acid B(C6F5)3. Isolated yields are indicated.

4128 DOI: 10.1021/acs.organomet.5b00546 Organometallics 2015, 34, 4127−4137 Organometallics Article

−3.5 and −4.0 ppm. With the exception of 2f,g, the 19F spectra revealed three signals in a 2:2:1 ratio due to the ortho (δ −132.7 to −133.3 ppm), meta (δ −164.9 to −165.9 ppm), and para (δ −160.1 to −161.2 ppm) fluorine atoms on the pentafluorophenyl rings. For 2f,g, an additional signal of intensity 1 was also observed at δ −105.5 and −105.2 ppm, respectively, due to the m-fluorine atoms on the phosphine. The 31P{1H} NMR spectra showed a signal due to the phosphorus environment which was dependent upon the δ − phosphine employed in the reaction: P(o-tol)3 ( 22.8 23.6 − δ − ppm (2b e)), P(m-FC6H5)3 ( 22.7 22.8 ppm (2f,g)), P(p- δ δ − OMeC6H5)3 ( 21.7 ppm (2h,i)), PBn3 ( 25.0 25.2 ppm (2j,k)). In the case of 2a−c,f,j,k crystals could be obtained from the CDCl3 solution. In addition, the stoichiometric reaction of 1b with B(C6F5)3 and P(o-tol)3 on a preparative scale in toluene followed by recrystallization from hexane/toluene yielded a small crop of colorless crystals. Single-crystal X-ray diffraction of the crystals 2a−c,f,j,k confirmed the identity of these species as the products arising from the addition of the Lewis acid/base pairs to the allene oxygen and β-carbon atom of the allene (Figures 1 and 2). The metric parameters for 2a−c,f,j,k are very similar (Table 1). Although the two double bonds C1C2 and C3C4 are conjugated, in the solid state the O−C1−C2−C3-C4 system is not completely planar with the C3−C4 bond twisting out of the O−C1−C2 plane, consistent with reduced delocalization of electron density across the π-system. The torsion angles between the C1C2C3 plane and the C2C3C4 plane are 29.5(3)° (2a), 23.8(5)° (2b), 23.8(3)° (2c), 14.2(4)° (2f), 22.5(3)° (2j), and 21.9(2)° (2k). This twisting effect presumably arises through crystal packing. Additionally, the aryl ring attached to C1 also rotates out of the O−C1−C2 plane, presumably also a result of steric hindrance. In all cases, it can be seen from the crystal structure that the two alkenes adopt an s-cis configuration. The Lewis acid/base addition reaction is thought to be initiated by adduct formation between the strong Lewis acid and the ketone oxygen atom through a σ-activation mode. Our previous studies on the reactivity of B(C6F5)3 with related propargyl amides and esters revealed that adduct formation occurred immediately, albeit reversibly, under ambient conditions.8,9 This O-coordination has the effect of removing electron density from the β-position of the allene, resulting in the buildup of a large partial positive charge at this carbon atom. This renders the β-position susceptible to nucleophilic attack (conjugate addition) from the phosphine resulting in 2a−k (Scheme 4). The independent reaction of phosphines Figure 1. Solid-state molecular structures of 2a−c,f (from top to with the allenes in the absence of B(C6F5)3 showed no reaction, 31 31 1 bottom): C, black; O, red; B, yellow-green; F, pink; P, orange. as evidenced by the P and P{ H} NMR spectra, which only Hydrogen atoms are omitted for clarity. show free phosphine. This does not rule out addition of the phosphine to the allene which may be rapid and reversible at room temperature but rather suggests that B(C6F5)3 is -rich allenes nucleophilic attack of the central carbon atom onto 23 necessary for the isolation of the addition product. B(C6F5)3 but did not observe any carboboration due to the Reactivity of Allenyl Ketones and Esters toward enhanced stability of the allene−borane products generated.22 B(C6F5)3. The study of the intrinsic reactivity of allene Products 3a−e were identified as α,β-unsaturated ketones with fl β substrates with B(C6F5)3 in the absence of added Lewis base a penta uorophenyl group at the -position (Scheme 5). The was subsequently investigated. The 1:1 stoichiometric reactions reactions were selective, giving products in which the double- − fi of the allenes 1a e with B(C6F3)3 proceeded very rapidly at bond con guration is exclusively E. The products were fully room temperature (<30 min) to give new compounds in which characterized by multinuclear NMR, mass spectrometry, and/or 11 the B(C6F5)3 underwent a regioselective 1,2-carboboration elemental analysis. In the B NMR spectrum a single broad reaction with the terminal CC moiety of the allene. peak observed around 0 ppm is attributed to coordination of Interestingly, Alcarazo et al. observed in the case of electron the borane to the ketone oxygen atom, yielding an intra-

4129 DOI: 10.1021/acs.organomet.5b00546 Organometallics 2015, 34, 4127−4137 Organometallics Article

Scheme 4. Reactions of Allenes with Lewis Acid/Base Pairs

− a Scheme 5. Reactions of B(C6F5)3 with Allenes 1a e

Figure 2. Solid-state molecular structures of 2j,k (from top to bottom): C, black; O, red; B, yellow-green; F, pink; P, orange; Cl, aquamarine. Hydrogen atoms are omitted for clarity. molecular chelate analogous to those observed previously for related allyl boron products.9 In the 19F NMR spectrum six ff peaks are observed for the two di erent C6F5 rings in a 2:1 ratio. Crystals of 3a,b suitable for X-ray diffraction could be aIsolated yields are indicated. obtained by dissolving the solid in a few drops of toluene with the addition of 1 drop of hexane and maintaining the Unlike the reactions of 1 with B(C6F5)3, which were rapid at temperature at −40 °C. X-ray diffraction analysis confirmed room temperature, the 1:1 reactions of the allenes 4a,b with α β the products as being , -unsaturated ketones in which a C6F5 B(C6F5)3 were inconclusive by in situ NMR in CDCl3. β group has been transferred to the -position of the allene and However, the preparative reaction in CH2Cl2 showed evidence the boron transferred to the γ-positioninanet1,2- of the 1,2-carboboration products 5a,b, respectively, which carboboration reaction (Figure 3). The structural parameters were formed as a racemic mixture (Scheme 6). Slow for the C4OB heterocycle in the products 3a,b are the same evaporation of volatiles from a hexane/CH2Cl2 (5a)or within error and are included in Table 2. hexane/toluene (5b) solution afforded a very small crop of Further proof of the identity of these compounds was colorless crystals suitable for single-crystal X-ray diffraction obtained from the reactions of the allenyl esters 4a,b with (Figure 4). fi B(C6F3)3 (Scheme 6). These allenes contain ester function- In the case of 5a,b, the structural analysis con rmed that π alities rather than ketones as in the allenes 1 and could be easily B(C6F5)3 had added in a 1,2-fashion across the -bond of the fi prepared in one step in a modi ed Wittig reaction with the allene distant from the ester group, with one of the C6F5 groups corresponding acyl chlorides (see the Supporting Information). being transferred from boron to carbon. These reactions are

Table 1. Experimental Bond Lengths (Å) in 2a−c,f,j,k

bond 2a 2b 2c 2f 2j 2k C1−C2 1.347(2) 1.352(4) 1.356(2) 1.356(3) 1.359(3) 1.353(2) C2−C3 1.465(3) 1.468(4) 1.464(2) 1.460(3) 1.463(3) 1.466(2) C3−C4 1.337(3) 1.332(5) 1.340(2) 1.339(3) 1.337(3) 1.335(2) C1−O 1.339(2) 1.334(4) 1.336(2) 1.337(3) 1.339(2) 1.335(2) O−B 1.495(3) 1.493(4) 1.489(2) 1.499(3) 1.490(2) 1.493(2) C3−P 1.818(2) 1.825(3) 1.811(1) 1.811(2) 1.800(2) 1.804(1)

4130 DOI: 10.1021/acs.organomet.5b00546 Organometallics 2015, 34, 4127−4137 Organometallics Article

Scheme 6. Reactions of B(C6F5)3 with Allenyl Esters 4.

Figure 3. Solid-state molecular structures of 3a (top) and 3b (bottom, toluene solvent omitted for clarity): C, black; O, red; B, yellow-green; F, pink. Hydrogen atoms are omitted for clarity.

β regioselective, with the C6F5 group being transferred to the - position of the allene and the B(C6F5)2 group being installed at the γ-position. The reactions were also selective, giving the E isomer, which is presumably a result of the intramolecular nature of the reaction and due to the formation of a stable six- Figure 4. Solid-state molecular structures of 5a (top, only one of the two independent molecules is shown) and 5b (bottom): C, black; O, membered chelate product via intramolecular coordination of red; B, yellow-green; F, pink. Hydrogen atoms are omitted for clarity. the ester oxygen to the Lewis-acidic boron center. In both cases 11 the compounds showed a broad peak in the B NMR spectrum Scheme 7. Reactions of B(C F ) with Allenyl Ester 4c at 4.2 ppm (5a) and 4.5 ppm (5b) typical for boron adducts. 6 5 3 The structural parameters are summarized in Table 2. However, the reaction outcome was altered by traces of water in the mixture of allenyl ester 4c and B(C6F5)3. In this case the corresponding γ-lactone cyclization product (6c)in which the lactone carbonyl oxygen atom coordinates to B(C6F5)3 was obtained in 56% yield (Scheme 7). This compound formulation was veriﬁed by single-crystal X-ray trigger these cyclization reactions in a catalytic fashion are diﬀraction (Figure 5) and resembles compounds typically ongoing but are challenging, as complete suppression of the obtained in catalytic reactions with π-Lewis acidic transition 1,2-carboboration pathway is required in order to promote metals such as gold and palladium.24,25 Further attempts to lactone formation.

a Table 2. Experimental Bond Lengths in 3a,b and 5a,b

5a bond 3a 3b 5b C1−C2 1.437(4) 1.440(3) 1.4661(3) 1.4636(3) 1.4655(16) C2−C3 1.352(3) 1.355(2) 1.3416(3) 1.3482(3) 1.3565(16) C3−C4 1.496(3) 1.497(3) 1.5190(3) 1.5180(3) 1.5235(17) C1−O 1.268(3) 1.277(2) 1.2554(2) 1.2557(2) 1.2613(15) O−B 1.569(3) 1.565(2) 1.5732(3) 1.5723(3) 1.5660(16) B−C4 1.604(3) 1.603(3) 1.6183(3) 1.6148(3) 1.6136(17) a5a has two molecules in the asymmetric unit.

4131 DOI: 10.1021/acs.organomet.5b00546 Organometallics 2015, 34, 4127−4137 Organometallics Article

in situ 11B NMR spectrum in the 1:1 reaction of allenyl ester 4c and B(C6F5)3. This might be attributed to the generation of the vinyl borate compound V (in the cyclization of propargyl amides the corresponding vinyl borate compounds exhibited very similar chemical shifts in the 11B NMR spectra δ ∼−16 ppm).8,23 In the case where water is present, hydrolysis of V takes place releasing the γ-lactone 6 via protodeboronation of the C−B bond and dealkylation of the oxygen atom.26 The presence of water appears necessary to generate a driving force for the cyclization event, as in its absence conversion to the vinyl borate could not be observed. This suggests a reversible cyclization process, which can only be trapped by the presence Figure 5. Solid-state structure of compound 6c: C, black; O, red; B, of a suitable nucleophile which acts to dealkylate the oxygen yellow-green; F, pink. Hydrogen atoms are omitted for clarity. atom generating the B(C6F5)3-substituted lactone. Subsequent γ protodeboronation regenerates B(C6F5)3 and yields the - Mechanistically, we attribute the generation of the 1,2- lactone. carboboration and cyclization products to two different The alternative bond activation process leading to the 1,2- potential activation modes: σ-activation (dative bond formation carboboration product is triggered by the σ-activation of the between the carbonyl oxygen atom and boron) and π-activation carbonyl moiety. Here, adduct formation between the Lewis (Scheme 8). It is possible that a weak van der Waals interaction acid and the ketone oxygen atom reduces the electron density from the β-position of the allene moiety, generating a partial Scheme 8. Proposed Mechanism for σ vs π Activation of positive charge while at the same time increasing the − Allenyl Esters nucleophilicity of the B C6F5 bond (I and II). This facilitates β migration of the C6F5 group to the -position of the allene, yielding the dienyloxyborane III which undergoes a [1,5]-B sigmatropic shift to give the products 3/5. The diastereose- lective formation of the double bond in these reactions is presumably due to the formation of a six-membered intramolecular chelate that dictates the geometry of the double bond in the product. This mechanism is also in agreement with the observed selectivity: while the allenyl ketones coordinate more strongly to B(C6F5)3 and therefore strongly polarize the allene moiety, the 1,2-carboboration mechanism overpowers other reaction pathways. In contrast, the corresponding allenyl esters form much weaker adducts;27 therefore, alternative reaction pathways become accessible. In the absence of water 1,2-carboboration products 5 are formed, while in its presence the cyclization pathway yielding 6 can dominate. Theoretical Considerations. Two sets of representative DFT calculations were undertaken to probe the activity of these acceptor-substituted allenes with respect to (i) addition of a Lewis acid/base pair and (ii) addition of the Lewis acid fi followed by a subsequent C6F5 migration. In the rst instance addition of B(C6F5)3 to the parent allenyl ketone 1a was considered followed by addition of triphenylphosphine. In each case the structures were fully optimized at the B3LYP/6-31G* level of theory, followed by thermodynamic calculations based on single-point B3LYP/6-311G*+ computations which were ZPE corrected. Furthermore, a NBO analysis was carried out to probe changes to the bonding scenario. These reveal that σ- coordination of the Lewis acid to the allenyl ketone (Lewis acid/base coordination) is thermodynamically favorable (ΔH = −23 kJ/mol) and leads to an increase in partial charge on the between the borane and π-bond occurs prior to 1,2-addition, as central carbon atom of the allene from +0.12 to +0.21 e. 11 observed in FLP chemistry, although another possible Subsequent addition of PPh3 is also thermodynamically explanation would be the formation of a covalently bound favorable (ΔH = −55 kJ/mol). The geometric parameters for borate−carbocation intermediate. In the case of alkene−borane the final geometry-optimized structure are in good agreement π-activation of the allene moiety (IV) a vinyl borate compound with experimental data. Similarly, studies were undertaken to is formed (V). This reactivity is analogous to that observed in probe the activation of the allenyl ester MeCHC 25 transition-metal-catalyzed reactions. Although a pathway CMeCO2Me (4d) by B(C6F5)3 followed by C6F5 migration. involving the B(C6F5)3-mediated generation of a strong These revealed that initial adduct formation was also favorable + Brønsted acid (H ) in the presence of water cannot be by 12 kJ/mol and that C6F5-group migration was favored by excluded, we observe a small sharp peak at δ −15.8 ppm in the 111 kJ/mol. Attempts to identify a concerted process in which

4132 DOI: 10.1021/acs.organomet.5b00546 Organometallics 2015, 34, 4127−4137 Organometallics Article − C6F5 migration occurred concomitantly with C B bond Synthesis of 2. General Procedure. PR3 (0.1 mmol, 1 equiv) in formation proved unsuccessful, suggesting a multistep migra- CDCl3 (0.5 mL) was added to B(C6F5)3 (51 mg, 0.1 mmol, 1 equiv) tion process with one or more transient intermediates. An NBO to give a pale yellow solution, which was then transferred to 1 (0.1 mmol, 1 equiv) to give a red solution. The mixture was transferred to analysis of the coordination of the Lewis acid to the allenyl 1 13 11 19 31 31 1 ester (4d) revealed that while partial charge on the central an NMR tube, and the H, C B, F, P and P{ H} NMR spectra were measured. Multinuclear NMR reaction spectra for 2a were not carbon center of the allene increased upon coordination (from recorded due to the product precipitating out of solution upon +0.13 to +0.16 e), this increase in partial positive charge was reaction; similarly, an in situ 13C NMR spectrum of 2c was not much less pronounced (almost negligible) than that observed recorded for the same reason. μ −1 ν with allenyl ketones (see above). This is in accordance with the Compound 2a. Yield: 58.3 mg, 88.9 mol, 89%. IR (cm ): max observations that the allenyl ketones undergo much more rapid 3054, 2987, 2685, 2412, 2305, 1642, 1558, 1515, 1466, 1423, 1266, 1087, 980, 896, 747, 735, 706. HRMS ESI−: m/z calculated for [M − C6F5-group transfer than the corresponding allenyl esters (see − − the Supporting Information). H] [C49H28BF15OP] , 958.1768; found, 958.1782. Compound 2b. Yield: 65 mg, 66.7 μmol, 67%. Mp: 178−193 °C. 1 − ■ CONCLUSIONS H NMR (500 MHz, CDCl3, 298 K): 7.37 7.07 (m, 12H, phosphine − − − 3 − C H), 7.37 7.07 (m, 4H, Ar H), 5.40 (d, JHP = 53 Hz, 1H, C H), Addition of a (frustrated) Lewis acid−base pair to acceptor- 3 − 3  5.22 (d, JHP = 26 Hz, 1H, C H), 4.34 (d, JHP = 11 Hz, 1H, C CH) substituted allenes (allenyl ketones or esters) occurs via O − 2.45 (s, br, 3H, Ar CH3), 2.25 (s, br, 3H, phosphine CH3), 1.78 (s, br, 13 coordination of the ketone to the Lewis acid with the associated 3H, phosphine CH3), 1.59 (s, br, 3H, phosphine CH3). C NMR activation of the central allene carbon atom toward nucleophilic (126 MHz, CDCl3, 298 K): 165.2 (s), 151.5 (s), 148.9 (s), 146.9 (s), attack by the phosphine. In the absence of a Lewis base two 138.4 (s), 137.5 (s), 134.9 (s), 133.6 (s), 132.2 (s), 130.9 (s), 129.7 reaction pathways need to be discriminated: σ-oxygen (s), 127.7 (s), 93.7 (s), 22.6 (s), 21.5 (s). 11B NMR (160 MHz, − 19 activation vs π-allene activation. While in the case of the CDCl3, 298 K): 4.0 (s, br). F NMR (282 MHz, CDCl3, 298 K): − − 3 − 132.7 (m, 2F, o-F), 161.1 (t, JFF = 20 Hz, 1F, p-F), 165.9 (m, 2F, strongly polarized and strongly coordinating allenyl ketones 31 31 1 m-F). P NMR (202 MHz, CDCl3, 298 K): 22.9 (s, br). P{ H} exclusive σ-activation triggers a 1,2-carboboration mechanism, − NMR (202 MHz, CDCl , 298 K): 23.0 (s). IR (cm 1): ν 3055, allenyl esters can undergo alternative reaction pathways. In the 3 max γ 2987, 2925, 2853, 2687, 2412, 2306, 1643, 1586, 1558, 1513, 1465, presence of water the formation of -lactone products could be 1423, 1390, 1373, 1266, 1202, 1137, 1114, 1087, 997, 980, 927, 897, ff − − observed, which is reminiscent of reactivity typically e ected by 836, 809, 742, 707. HRMS ESI : m/z calculated for [M − H] π − -Lewis acidic transition metals. We should note that the 1,2- [C50H30BF15OP] , 972.1924; found, 972.1971. Anal. Calcd for carboboration products of the reactions with B(C6F5)3 generate C50H31BF15OP: C, 61.62; H, 3.21. Found: C, 61.68; H, 3.14. organoboron compounds, which might have applications in Compound 2c. Yield: 47 mg, 47.4 μmol, 47%. Mp: 176−187 °C. 1 − organic synthesis. Future studies will aim at addressing their H NMR (500 MHz, CDCl3, 298 K): 8.28 7.14 (m, 12H, phosphine − 3 3 reactivity toward organic electrophiles as well as investigating C H), 7.52 (d, JHH = 8.7 Hz, 2H, o-H), 6.89 (d, JHH = 8.7 Hz, 2H, 3 − 3 the catalytic synthesis of γ-lactones using B(C F ) . m-H), 5.54 (d, JHP = 52 Hz, 1H, C H), 5.35 (d, JHP = 26 Hz, 1H, 6 5 3 − 3  − − C H), 4.42 (d, JHP = 11 Hz, 1H, C C H), 3.79 (s, 3H, Ar ■ EXPERIMENTAL SECTION OCH3), 2.45 (s, br, 3H, phosphine CH3), 1.85 (s, br, 3H, phosphine 11 CH3), 1.65 (s, br, 3H, phosphine CH3). B NMR (160 MHz, CDCl3, General Experimental Considerations. With the exception of − 19 − 298 K): 4.0 (s, br). F NMR (282 MHz, CDCl3, 298 K): 132.7 the synthesis of starting materials, all reactions and manipulations were − 3 − (m, 2F, o-F), 161.1 (t, JFF = 21 Hz, 1F, p-F), 165.9 (m, 2F, m-F). carried out under an atmosphere of dry, O2-free nitrogen using 31 31 1 P NMR (202 MHz, CDCl3, 298 K): 22.7 (s, br). P{ H} NMR (202 standard double-manifold techniques with a rotary oil pump. An −1 ν fi MHz, CDCl3, 298 K): 22.8 (s). IR (cm ): max 3055, 2988, 2931, argon- or nitrogen- lled glovebox (MBRAUN) was used to 2856, 2687, 2524, 2411, 2306, 1713, 1643, 1610, 1587, 1557, 1513, manipulate solids, including the storage of starting materials, room- 1465, 1423, 1390, 1373, 1267, 1175, 1138, 1088, 1032, 980, 925, 908, temperature reactions, product recovery, and sample preparation for − 897, 842, 808, 756, 724, 706. HRMS ESI : m/z calculated for [M − analysis. All solvents (toluene, CH Cl , pentane, hexane) were dried by − − 2 2 H] [C H BF O P] , 988.1873; found, 988.1891. Anal. Calcd for employing a Grubbs-type column system (Innovative Technology) or 50 30 15 2 C H BF O P·CDCl : C, 55.14; H, 2.99. Found: C, 55.07; H, 2.26. an MB SPS-800 solvent purification system and stored under a 50 31 15 2 3 Compound 2d. Yield: 98 mg, 98.5 μmol, 99%. Mp: 186−194 °C. nitrogen atmosphere. Deuterated solvents were distilled and/or dried 1 − H NMR (500 MHz, CDCl3, 298 K): 8.15 7.17 (m, 12H, phosphine over molecular sieves before use. Chemicals were purchased from − 3 3 1 13 11 19 C H), 7.53 (d, JHH = 8.5 Hz, o-H), 7.34 (d, JHH = 8.5 Hz, m-H), commercial suppliers and used as received. H, C, B, and F NMR 3 − 3 − 5.52 (d, JHP = 52 Hz, 1H, C H), 5.38 (d, JHP = 25 Hz, 1H, C H), spectra were recorded on Bruker Avance II 400, Bruker Avance 500, 3 − 4.48 (d, JHP = 10 Hz, 1H, C H), 2.44 (s, br, 3H, phosphine CH3), Bruker Avance III 600, and JEOL 300 spectrometers. Chemical shifts 13 are expressed as parts per million (ppm, δ) downfield of 1.83 (s, br, 3H, phosphine CH3), 1.66 (s, br, 3H, phosphine CH3). C tetramethylsilane (TMS) and are referenced to CDCl (7.26/77.16 NMR (126 MHz, CDCl3, 298 K): 163.8 (m), 149.1 (s), 147.2 (s), 3 143.9 (s), 143.2 (s), 143.2 (s), 139.0 (s), 137.8 (s), 135.3 (s), 134.7 ppm) and CD2Cl2 (5.32/53.80 ppm) as internal standards. NMR 19 31 · (s), 134.0 (s), 133.5 (s), 130.0 (s), 129.7 (s), 129.5 (s), 129.3 (s), spectra were referenced to CFCl3 ( F), H3PO4 ( P) and BF3 Et2O/ CDCl (11B). The description of signals include the following: s = 126.6 (s), 113.5 (s), 94.6 (m), 26.0 (s), 22.8 (s, br), 21.5 (s), 19.4 (s). 3 11 − 19 singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broad. B NMR (160 MHz, CDCl3, 298 K): 3.9 (s, br). F NMR (282 − − 3 All coupling constants are absolute values, and J values are expressed in MHz, CDCl3, 298 K): 132.9 (m, 2F, o-F), 160.8 (t, JFF = 20 Hz, 13 1 − 31 hertz (Hz). C NMR spectra were all measured as H-decoupled. IR 1F, p-F), 165.6 (m, 2F, m-F). P NMR (202 MHz, CDCl3, 298 K): 31 1 spectra were recorded using a Shimadzu IRAffinity-1 spectropho- 23.0 (s, br). P{ H} NMR (202 MHz, CDCl3, 298 K): 23.1 (s). IR −1 ν tometer, Elemental analysis was conducted by Mr. Stephen Boyer of (cm ): max 3055, 2987, 2685, 2412, 2308, 1644, 1585, 1556, 1515, London Metropolitan University. Mass spectra were obtained in house 1488, 1466, 1422, 1390, 1371, 1296, 1267, 1202, 1171, 1136, 1122, using a Waters LCT Premier/XE or a Waters GCT Premier 1088, 1032, 1016, 996, 979, 926, 910, 898, 844, 808, 750, 729, 706. 10 − − − − spectrometer and were calculated using B. All spectra were analyzed HRMS ESI : m/z calculated for [M H] [C49H27BClF15OP] , assuming a first-order approximation. For the synthesis of the allene 992.1378; found, 992.1411. starting materials (1) and the reactions of 1 with phosphines along Compound 2e. Yield: 100 mg, 88.1 μmol, 88%. Mp: 98−115 °C. 1 11 13 19 31 31 1 1 − with H, B, C, F, P, and P{ H} NMR spectra for all H NMR (500 MHz, CDCl3, 298 K): 7.59 6.84 (m, 12H, phosphine compounds see the Supporting Information. C−H), 7.59−6.84 (m, 4H, Ar−H), 7.59−6.84 (m, br, 2H, furan C−

4133 DOI: 10.1021/acs.organomet.5b00546 Organometallics 2015, 34, 4127−4137 Organometallics Article

3 − 3 − H), 5.87 (d, JHP = 50 Hz, 1H, C H), 5.63 (d, JHP = 23 Hz, 1H, C 1505, 1464, 1443, 1423, 1364, 1297, 1267, 1184, 1170, 1112, 1084, 3  − − H), 4.36 (d, JHP = 7.4 Hz, 1H, C C H), 2.30 (s, 3H, phosphine 1026, 966, 897, 834, 805, 753, 728, 705. HRMS ESI : m/z calculated 13 − − − CH3), 1.79 (s, 3H, phosphine CH3), 1.58 (s, 3H, phosphine CH3). C for [M H] [C50H30BF15O5P] , 1036.1721; found, 1036.1725. μ − ° 1 NMR (126 MHz, CDCl3, 298 K): 151.5 (s), 150.7 (m), 148.3 (s), Compound 2i. Yield: 100 mg, 95.9 mol, 96%. Mp: 87 99 C. H 3 147.7 (s), 145.8 (s), 138.6 (s), 136.5 (s), 133.9 (s), 129.8 (s), 128.1 NMR (500 MHz, CDCl3, 298 K): 7.30 (d, JHH = 8.8 Hz, 2H, Ar o-H), − − 3 (s), 127.4 (s), 126.8 (s), 125.3 (s), 114.1 (s), 112.2 (s), 95.2 (m), 21.6 7.24 7.20 (m, 6H, phosphine C H), 7.12 (d, JHH = 8.4 Hz, 2H, Ar 11 − 19 − − 3 (s), 20.0 (m). B NMR (160 MHz, CDCl3, 298 K): 3.6 (s, br). F m-H), 6.92 6.91 (m, 6H, phosphine C H), 5.47 (d, JHP = 49 Hz, − − 3  − 3  − 3 NMR (282 MHz, CDCl3, 298 K): 132.9 (m, 2F, o-F), 161.2 (t, JFF 1H, C H), 5.06 (d, JHP = 23 Hz, 1H, C H), 4.54 (d, JHP = 8.6 − 31  13 = 21 Hz, 1F, p-F), 165.7 (m, 2F, m-F). P NMR (202 MHz, CDCl3, Hz, 1H, CH), 3.78 (s, 9H, phosphine CH3). C NMR (126 MHz, 31 1 298 K): 23.7 (s), 19.7 (s, br). P{ H} NMR (202 MHz, CDCl3, 298 CDCl3, 298 K): 164.6 (m), 148.8 (m), 146.7 (m), 136.0 (m), 135.5 −1 ν K): 23.6 (s), 19.7 (s, br). IR (cm ): max 3055, 2988, 2686, 2522, (m), 129.8 (s), 129.1 (s), 128.9 (s), 128.5 (s), 128.1 (s), 125.2 (s), 2412, 2306, 1712, 1645, 1593, 1566, 1555, 1514, 1466, 1424, 1368, 116.6 (m), 115.7 (m), 108.6 (m), 107.9 (m), 55.6 (m), 31.5 (s), 22.6 11 − 1265, 1223, 1170, 1137, 1087, 1025, 978, 897, 808, 753, 723, 705. (s), 21.3 (s), 14.0 (s). B NMR (160 MHz, CDCl3, 298 K): 3.8 (s, − − − − 19 − HRMS ESI : m/z calculated for [M H] [C53H29BClF15O2P] , br). F NMR (282 MHz, CDCl3, 298 K): 133.1 (m, 2F, o-F), − 3 − 31 1058.1484; found, 1058.1536. 160.9 (t, JFF = 20.7 Hz, 1F, p-F), 165.6 (m, 2F, m-F). P NMR μ − ° 1 31 1 Compound 2f. Yield: 55 mg, 54.9 mol, 55%. Mp: 151 164 C. H (202 MHz, CDCl3, 298 K): 21.6 (s). P{ H} NMR (202 MHz, − − −1 ν NMR (500 MHz, CDCl3, 298 K): 7.71 7.48 (m, 3H, phosphine C CDCl3, 298 K): 21.7 (s). IR (cm ): max 3054, 2988, 2846, 2685, 3 − 3 H), 7.39 (t, br, JHH = 8.1 Hz, 3H, phosphine C H), 7.34 (d, JHH = 2309, 1713, 1644, 1594, 1567, 1514, 1504, 1465, 1422, 1366, 1300, 8.2 Hz, 2H, o-H), 7.16 (m, 3H, phosphine C−H), 7.05 (m, 3H, 1265, 1223, 1184, 1113, 1088, 1024, 979, 965, 896, 835, 806, 739, 706. − 3 3 − − − − phosphine C H), 6.74 (d, JHH = 8.2 Hz, 2H, m-H), 5.63 (d, JHP =53 HRMS ESI : m/z calculated for [M H] [C49H27BClF15O4P] ,  − 3  − 3 Hz, 1H, C H), 5.06 (d, JHP = 25 Hz, 1H, C H), 4.45 (d, JHP = 1040.1225; found, 1040.1245.  − 13 1 9.8 Hz, 1H, C H), 3.67 (s, 3H, OCH3). C NMR (126 MHz, Compound 2j. Yield: 62 mg, 62.6 μmol, 63%. Mp: 168−181 °C. H − − CDCl3, 298 K): 166.0 (m), 163.8 (m), 161.9 (m), 160.0 (s), 148.6 (s), NMR (500 MHz, CDCl3, 298 K): 7.39 7.34 (m, 9H, phosphine C 139.6 (s), 137.5 (m), 135.4 (m), 132.9 (s), 132.6 (m), 131.1 (s), 129.7 H), 7.39−7.34 (m, 2H, Ar o-H) 6.89 (m, 6H, phosphine C−H), 6.80 3 3  − (s), 129.7 (s), 129.4 (s), 123.1 (m), 120.6 (m), 119.4 (s), 118.8 (s), (d, JHH = 8.8 Hz, 2H, Ar m-H), 5.58 (d, JHP = 47 Hz, 1H, C H), 11 3  − 3  114.0 (s), 92.3 (m), 55.1 (s). B NMR (160 MHz, CDCl3, 298 K): 5.19 (d, JHP = 20 Hz, 1H, C H), 4.87 (d, JHP = 8.3 Hz, 1H, − 19 − 3 3.8 (s, br). F NMR (282 MHz, CDCl3, 298 K): 105.5 (m, 1F, CH), 3.78 (s, 3H, OCH3), 3.20 (d, JHP = 13 Hz, 6H, phosphine − 3 3 13 phosphine), 133.2 (d, JFF = 22 Hz, 2F, o-F), 160.8 (t, JFF = 21 Hz, CH2). C NMR (126 MHz, CDCl3, 298 K): 196.9 (s), 164.5 (m), 3 31 1F, p-F), 165.6 (t, JFF = 21 Hz, 2F, m-F). P NMR (202 MHz, 149.1 (s, br) 147.2 (s, br) 137.9 (s, br), 137.3 (s), 135.9 (m), 134.7 31 1 CDCl3, 298 K): 22.6 (s). P{ H} NMR (202 MHz, CDCl3, 298 K): (s), 133.3 (s), 130.3 (m), 129.2 (m), 128.7 (m), 126.1 (s), 91.3 (m), −1 ν 11 − 22.7 (s). IR (cm ): max 3055, 2987, 2686, 2411, 2306, 1644, 1610, 28.6 (m), 26.0 (m). B NMR (160 MHz, CDCl3, 298 K): 3.57 (s, 19 − 1585, 1556, 1514, 1480, 1465, 1423, 1270, 1262, 1237, 1175, 1098, br). F NMR (282 MHz, CDCl3, 298 K): 132.9 (m, 2F, o-F), + − 3 31 1090, 1033, 999, 981, 925, 897, 844, 794, 757, 723, 706. HRMS AP : 160.3 (t, JFF = 21.1, 1F, p-F), 165.1 (m, 2F, m-F). P NMR (202 + + 31 1 m/z calculated for [M] [C47H22BF18O2P] , 1002.1163; found, MHz, CDCl3, 298 K): 25.1 (m). P{ H} NMR (202 MHz, CDCl3, −1 ν 1002.1166. Anal. Calcd for C47H22BF18O2P: C, 56.31; H, 2.21. 298 K): 25.0 (s). IR (cm ): max 3055, 2987, 2961, 2926, 2855, 2687, Found: C, 56.27; H, 2.18. 2411, 2359, 2308, 1716, 1643, 1609, 1588, 1565, 1514, 1498, 1465, Compound 2g. Yield: 93 mg, 92.4 μmol, 92%. Mp: 119−141 °C. 1423, 1365, 1266, 1175, 1088, 1032, 980, 925, 896, 865, 845, 808, 751, 1 − − − H NMR (500 MHz, CDCl3, 298 K): 7.54 7.02 (m, 12H, phosphine 730, 706. HRMS ESI : m/z calculated for [M − H] − − − 3  − − C H) 7.54 7.02 (m, 4H, Ar H), 5.62 (d, JHH = 53 Hz, 1H, C [C50H30BF15O2P] , 988.1873; found, 988.1896. Anal. Calcd for 3  − 3 H), 5.11 (d, JHH = 25 Hz, 1H, C H), 4.52 (d, JHH = 9.4 Hz, 1H, C50H31BF15O2P: C, 60.63; H, 3.15. Found: C, 60.48; H, 2.94.  13 1 CH). C NMR (126 MHz, CDCl3, 298 K): 165.3 (m), 164.4 (m), Compound 2k. Yield: 87 mg, 87.4 μmol, 87%. Mp: 91−103 °C. H − − 162.3 (m), 149.1 (m), 147.2 (m), 140.2 (m), 137.9 (m), 137.7 (s), NMR (500 MHz, CDCl3, 298 K): 7.29 7.21 (m, 9H, phosphine C 135.8 (m), 135.2 (s), 134.2 (m), 133.1 (m), 130.8 (m), 130.0 (m), H), 7.29−7.21 (m, 2H, Ar o-H), 7.14 (m, 2H, Ar m-H), 6.77 (m, 6H, − 3 − 3 129.4 (m), 127.9 (s), 127.3 (s), 123.8 (m), 121.2 (m), 119.7 (m), phosphine C H), 5.42 (d, JHP = 47 Hz, 1H, C H), 5.09 (d, JHP =20 11 − 3  3 119.0 (m), 117.0 (m), 93.6 (m). B NMR (160 MHz, CDCl3, 298 K): Hz, 1, C H), 4.8 (d, JHP = 8.1 Hz, 1H, CH), 3.13 (d, JHP = 13 Hz, − 19 − 13 3.8 (s, br). F NMR (282 MHz, CDCl3, 298 K): 105.2 (m, 1F, 6H, phosphine CH2). C NMR (126 MHz, CDCl3, 298 K): 186.7 (s), − − 3 P(m-FC6H4)3), 133.3 (m, 2F, o-F), 160.4 (t, JFF = 20.3 Hz, 1F, p- 167.5 (s), 155.9 (s), 148.3 (s), 139.8 (s), 138.1 (s), 130.3 (m), 129.6 − 31 11 F), 165.4 (m, 2F, m-F). P NMR (202 MHz, CDCl3, 298 K): 22.9 (m), 128.8 (m), 126.2 (s), 93.6 (s), 34.5 (m), 22.4 (m). B NMR 31 1 − 19 (s, br). P{ H} NMR (202 MHz, CDCl3, 298 K): 22.8 (s). IR (160 MHz, CDCl3, 298 K): 3.5 (s, br). F NMR (282 MHz, CDCl3, −1 ν − − 3 (cm ): max 3055, 2988, 2964, 2932, 2874, 2861, 2686, 2411, 2307, 298 K): 132.9 (m, 2F, o-F), 160.1 (t, JFF = 20.6 Hz, 1F, p-F), − 31 1720, 1682, 1644, 1585, 1556, 1514, 1480, 1465, 1424, 1370, 1267, 164.9 (m, 2F, m-F). P NMR (202 MHz, CDCl3, 298 K): 25.4 (m). 1235, 1100, 1056, 1015, 998, 979, 966, 926, 898, 843, 794, 752, 731, 31P{1H} NMR (202 MHz, CDCl , 298 K): 25.2 (s). IR (cm−1): ν − − − − 3 max 707. HRMS ESI : m/z calculated for [M H] [C46H18BClF18OP] , 3055, 2987, 2927, 2854, 2687, 2412, 2361, 2307, 1714, 1678, 1645, 1004.0626; found, 1004.0672. 1589, 1515, 1464, 1423, 1364, 1266, 1222, 1088, 978, 964, 897, 751, μ − ° − − Compound 2h. Yield: 70 mg, 67.4 mol, 67%. Mp: 110 122 C. 728, 707. HRMS ESI : m/z calculated for [M − H] 1H NMR (500 MHz, CDCl , 298 K): 7.32 (d, 3J = 8.4 Hz, 2H, Ar o- − 3 HH [C49H27BClF15OP] , 992.1378; found, 992.1357. Anal. Calcd for 3 4 H), 7.24 (m, 6H, phosphine m-H), 6.89 (dd, JHH = 8.8 Hz, JHH = 2.5 C49H28BClF15OP: C, 59.15; H, 2.84. Found: C, 59.02; H, 2.86. 3 Hz, 6H, phosphine o-H), 6.70 (d, JHH = 8.7 Hz, 2H, Ar m-H), 5.47 (d, Alternative Synthesis of 2c. B(C6F5)3 (103 mg, 0.2 mmol), P(o- 3  − 3  − JHP = 50 Hz, 1H, C H), 5.00 (d, JHP = 24 Hz, 1H, C H), 4.47 tol)3 (60 mg, 0.2 mmol), and 1-(4-methoxyphenyl)buta-2,3-dien-1- 3  (d, JHP = 9.0 Hz, 1H, CH), 3.77 (s, 9H, phosphine CH3), 3.66 (s, one 1c (36 mg, 0.2 mmol) were mixed in toluene (2 mL), giving a 13 3H, OCH3). C NMR (126 MHz, CDCl3, 298 K): 164.8 (s), 164.11 yellow solution. The reaction mixture was left to stand at room (m), 159.9 (s), 149.0 (s, br), 147.1 (s, br), 137.7 (m), 135.8 (m), temperature for 24 h. Removal of the solvent aﬀorded an orange oil. 132.0 (s), 131.7 (s), 130.8 (s), 129.9 (s), 115.8 (m), 113.9 (s), 109.2 Washing with hexane (3 × 2 mL) yielded the product 2c (159 mg, (s), 108.4 (s), 94.0 (m), 55.7 (s), 55.3 (s). 11B NMR (160 MHz, 0.16 mmol, 80%). The addition of hot hexane (ca. 2 mL) and a few − 19 CDCl3, 298 K): 3.8 (s, br). F NMR (282 MHz, CDCl3, 298 K): drops of hot toluene to give a clear solution gave, upon cooling and − − 3 − 3 133.0 (m, 2F, o-F), 161.2 (t, JFF = 21 Hz, 1F, p-F), 165.8 (t, JFF prolonged storage (3−4 days), a few colorless needle-shaped crystals 31 = 21 Hz, 2F, m-F). P NMR (202 MHz, CDCl3, 298 K): 21.8 (s, br). of 2c. 31 1 −1 ν − P{ H} NMR (202 MHz, CDCl3, 298 K): 21.7 (s). IR (cm ): max Synthesis of Compounds 3a e. General Procedure. B(C6F5)3 3055, 2988, 2845, 2687, 2410, 2307, 1713, 1643, 1596, 1568, 1513, (51 mg, 0.1 mmol, 1 equiv) in CDCl3 (0.5 mL) was added to 3 (0.1

4134 DOI: 10.1021/acs.organomet.5b00546 Organometallics 2015, 34, 4127−4137 Organometallics Article

11 19 mmol, 1 equiv), the mixture was transferred to an NMR tube, and the (s), 20.5 (s). B NMR (160 MHz, CDCl3, 298 K): 1.9 (s, br). F 1 11 13 19 − − H, B, C, and F NMR spectra were measured. NMR (282 MHz, CDCl3, 298 K): 135.1 (m, 4F, o-F), 138.3 (m, 1 − 3 − 3 Compound 3a. Yield: 26 mg, 39.6 μmol, 40%. Mp: 48−51 °C. H 2F, o-F), 149.9 (t, JFF = 21 Hz, 1F, p-F), 158.1 (t, JFF = 21 Hz, 2F, 3 − − −1 ν NMR (500 MHz, CDCl3, 298 K): 8.26 (d, JHH = 7.7 Hz, 2H, o-H), p-F), 160.1 (m, 2F, m-F), 163.9 (m, 4F, m-F). IR (cm ): max 3 3 7.88 (t, JHH = 7.6 Hz, 1H, p-H), 7.67 (t, JHH = 7.7 Hz, 2H, m-H), 3055, 2987, 2686, 2411, 2361, 2343, 2307, 1714, 1651, 1519, 1493, − 13 7.33 (s, C H), 2.74 (s, 2H, CH2). C NMR (126 MHz, CDCl3, 298 1482, 1422, 1365, 1320, 1266, 1186, 1093, 1029, 982, 897, 752, 728, K): 192.6 (s), 169.1 (s), 148.9 (m), 147.0 (m), 146.3 (m), 143.2 (m), 706. Anal. Calcd for C32H9BClF15O2: C, 50.80; H, 1.20. Found: C, 141.4 (m), 141.0 (m), 138.6 (s), 138.3 (m), 137.3 (m), 136.4 (m), 50.74; H, 1.12. 11 132.9 (s), 131.9 (s), 130.1 (s), 121.2 (s), 116.0 (m), 27.2 (m). B Synthesis of 5a. B(C6F5)3 (103 mg, 0.2 mmol) was dissolved in 19 NMR (160 MHz, CDCl3, 298 K): 2.7 (s, br). F NMR (282 MHz, toluene (1 mL) and was added to ethyl 2,5-dimethylhexa-2,3-dienoate − − − CDCl3, 298 K): 135.1 (m, 4F, o-F), 138.2 (m, 2F, o-F), 149.1 (t, (4a; 34 mg, 0.2 mmol) in toluene (1 mL). The reaction mixture was 3 − 3 − JFF = 20.5 Hz, 1F, p-F), 157.6 (t, JFF = 20.7 Hz, 2F, p-F), 159.7 left to stand at room temperature for 24 h, giving an orange solution. − −1 ν ff (m, 2F, m-F), 163.6 (m, 4F, m-F). IR (cm ): max 3055, 2988, 2689, Removal of the solvent a orded a dark orange oil. Addition of hexane 2365, 2307, 1736, 1719, 1651, 1520, 1473, 1422, 1270, 1262, 1096, (2 mL) and a small amount of DCM (ca. 0.5 mL to solubilize the oil) 982, 897, 759, 721, 708. HRMS ESI−: m/z calculated for [M − H]− followed by slow evaporation of the solvent afforded colorless crystals − ff [C28H7BF15O] : ,654.0387; found, 654.0378. of the product. The remaining solution was decanted o and the solid Compound 3b. Yield: 64 mg, 95.5 μmol, 96%. Mp: 48−52 °C. 1H product washed with pentane (3 × 2 mL) and dried in vacuo to give 3 11 NMR (500 MHz, CDCl3, 298 K): 8.19 (d, JHH = 8.4 Hz, 2H, o-H), the pure product (36.3 mg, 27%, 0.05 mmol). B NMR (128 MHz, 3 − 7.46 (d, JHH = 8.4 Hz, 2H, m-H), 7.29 (s, C H), 2.71 (s, 2H, CH2), CDCl3, 298 K): 4.2 (s, br). 13 Synthesis of 5b. 2.54 (s, 3H, CH3). C NMR (126 MHz, CDCl3, 298 K): 191.8 (s), B(C6F5)3 (103 mg, 0.2 mmol) was dissolved in 168.1 (s), 151.7 (s), 149.2 (m), 147.4 (s), 145.5 (m), 143.5 (m), 141.3 toluene (1 mL) and was added to ethyl 2-methyl-5-phenylpenta-2,3- (m), 138.7 (m), 137.7 (m), 136.7 (m), 132.4 (s), 131.2 (s), 130.6 (s), dienoate (4b; 43 mg, 0.2 mmol) in toluene (1 mL), giving a yellow 11 121.4 (s), 116.4 (m), 22.8 (s). B NMR (160 MHz, CDCl3, 298 K): solution upon addition. The reaction mixture was left to stand at room 19 − ff 2.2 (s, br). F NMR (282 MHz, CDCl3, 298 K): 135.1 (m, 4F, o-F), temperature for 24 h. Removal of the solvent a orded a dark orange − − 3 − 138.5 (m, 2F, o-F), 149.6 (t, JFF = 20.1 Hz, 1F, p-F), 157.8 (t, oil. Addition of hexane (2 mL) and a small amount of toluene (ca. 0.5 3 − − JFF = 20.6 Hz, 2F, p-F), 159.9 (m, 2F, m-F), 163.7 (m, 4F, m-F). mL to solubilize the oil) followed by slow evaporation of the solvent − ff IR (cm 1): ν 3055, 2983, 2686, 2411, 2360, 2343, 2307, 1714, 1650, a orded colorless crystals of the product. The remaining solution was max ff × 1606, 1520, 1486, 1422, 1364, 1319, 1268, 1264, 1223, 1184, 1093, decanted o and the solid product washed with pentane (3 2 mL) 980, 896, 754, 726, 704. HRMS ESI−: m/z calculated for [M − H]− and dried in vacuo to give the pure product (13.7 mg, 9%, 0.02 mmol). − 1 − [C H BF O] , 668.0543; found, 668.0535. Anal. Calcd for H NMR (400 MHz, C6D6, 298 K): 7.08 7.07 (m, 3H, o-H and p-H), 29 9 15 3 C29H10BF15O: C: 51.97, H: 1.50. Found: C: 51.86, H: 1.46. 6.95 (d, JHH = 6.6 Hz, 2H, m-H), 4.68 and 4.43 (m, 2H, CH2 1 Compound 3c. Yield: 68 mg, 99.1 μmol, 99%. Mp: 64−71 °C. H diastereotopic), 3.08 and 2.65 (m, 2H, CH2 diastereotopic), 1.78 (d, 3 4 3 3 NMR (500 MHz, CDCl3, 298 K): 8.30 (d, JHH = 8.6 Hz, 2H, o-H), JHH = 2 Hz, 3H, CH3), 1.51 (t, JHH = 7.1 Hz, 3H, CH3), 0.88 (t, JHH − 3 11 7.23 (s, C H), 7.11 (d, JHH = 8.6 Hz, 2H, m-H), 4.00 (s, 3H, CH3), = 7.1 Hz, 1H, CH). B NMR (128 MHz, C6D6, 298 K): 4.5 (s, br). 13 19 − − 2.66 (s, 2H, CH2). C NMR (126 MHz, CDCl3, 298 K): 189.0 (s), F NMR (282 MHz, C6D6, 298 K): 132.9 (m, 2F, o-F), 133.2 (m, − − − 3 168.8 (s), 165.8 (s), 150.0 (m), 147.0 (m), 145.1 (m), 143.2 (m), 1F, o-F), 137.5 (br s, 2F, o-F), 139.3 (m, 1F, o-F), 152.2 (t, JFF = − 3 − 3 140.7 (m), 138.9 (m), 138.2 (m), 137.2 (m), 136.3 (m), 135.2 (s), 21 Hz, 1F, p-F), 157.2 (t, JFF = 21 Hz, 1F, p-F), 157.8 (t, JFF =20 125.5 (s), 120.9 (s), 116.4 (m), 115.8 (s), 56.4 (s), 26.9 (s, br). 11B Hz, 1F, p-F), −160.4 (m, 1F, m-F), −161.5 (m, 1F, m-F), −163.3 (m, 19 − NMR (160 MHz, CDCl3, 298 K): 1.7 (s, br). F NMR (282 MHz, 2F, m-F), 163.6 (m, 2F, m-F). − − − CDCl3, 298 K): 135.2 (m, 4F, o-F), 138.9 (m, 2F, o-F), 150.3 (t, Synthesis of 6. B(C6F5)3 (102 mg, 0.2 mmol) was dissolved in 3 − 3 − JFF = 20.9 Hz, 1F, p-F), 158.2 (t, JFF = 21.0 Hz, 2F, p-F), 160.2 toluene (2 mL) and added to ethyl 2,4-dimethylpenta-2,3-dienoate − −1 ν (m, 2F, m-F), 163.9 (m, 4F, m-F). IR (cm ): max 3056, 2989, 2687, (4c; 31 mg, 0.2 mmol) to give a brown solution. After the addition of 2413, 2361, 2308, 1713, 1649, 1593, 1572, 1518, 1496, 1465, 1427, 1 equiv of H2O the reaction mixture was left at room temperature for 1384, 1271, 1264, 1178, 1095, 1050, 1019, 993, 978, 922, 897, 845, 72 h. Slow evaporation of the solvent gave large brown crystals that 758, 722, 706. HRMS ESI−: m/z calculated for [M − H]− were suitable for X-ray diffraction. The remaining solid was washed − × [C29H9BF15O2] , 684.0605; found, 684.0624. with hexane (3 2 mL) and dried in vacuo to give the pure product 1 1 Compound 3d. Yield: 58 mg, 84.0 μmol, 84%. Mp: 61−66 °C. H (72 mg, 0.12 mmol, 56%). H NMR (500 MHz, CDCl3, 298 K): 7.42 3  13 NMR (500 MHz, CDCl3, 298 K): 8.22 (d, JHH = 8.5 Hz, 2H, o-H), (s, 1H, CH), 2.03 (s, 3H, CH3), 1.43 (s, 6H, (CH3)2). C NMR 3 − 7.64 (d, JHH = 8.5 Hz, 2H, m-H), 7.28 (s, 1H, C H), 2.75 (s, 2H, (126 MHz, CDCl3, 298 K): 160.9 (s), 148.8 (s), 146.8 (s), 138.9 (s), CH ). 13C NMR (126 MHz, CDCl , 298 K): 191.4 (s), 170.1 (s), 137.8 (s), 135.8 (s), 128.7 (s), 23.7 (s), 17.0 (s), 9.6 (s). 11B NMR 2 3 − 149.0 (m), 147.0 (m), 146.1 (s), 145.3 (m), 143.3 (m), 141.0 (m), (160 MHz, CDCl3, 298 K): 40.4 (s, br), 26.2 (s, br), 1.1 (s, br, B O − 19 − 139.4 (m), 138.4 (m), 137.3 (m), 136.3 (m), 133.0 (s), 131.2 (s), adduct), 3.6 (s, br). F NMR (282 MHz, CDCl3, 298 K): 134.7 3 − 3 − 130.6 (s), 120.8 (s), 115.9 (m), 114.3 (s), 76.2 (s), 27.3 (s), 24.8 (s), (d, JFF = 21 Hz, 2F, o-F), 157.3 (t, JFF = 21 Hz, 1F, p-F), 164.2 (t, 11 19 3 + − 21.1 (m). B NMR (160 MHz, CDCl3, 298 K): 2.1 (s, br). F NMR JFF = 21 Hz, 2F, m-F). HRMS ESI : m/z calculated for [M − − + + (282 MHz, CDCl3, 298 K): 135.2 (m, 4F, o-F), 138.0 (m, 2F, o-F), B(C6F5)3] [C7H10O2] , 126.0681; found, 126.0683. − 3 − 3 148.7 (t, JFF = 21.1 Hz, 1F, p-F), 157.4 (t, JFF = 20.7 Hz, 2F, p-F), Crystallographic Studies. Crystallographic studies of compounds − − −1 ν 159.6 (m, 2F, m-F), 163.5 (m, 4F, m-F). IR (cm ): max 3055, 2, 3, and 6 were undertaken on single crystals mounted in Paratone 2988, 2686, 2362, 2343, 2307, 1712, 1669, 1651, 1520, 1484, 1423, and studied on an Agilent SuperNova dual four-circle diffractometer 1363, 1320, 1266, 1223, 1093, 980, 896, 749 732, 706. HRMS ESI−: using monochromatic Cu Kα (1.54184 Å) or Mo Kα (0.71073 Å) − − − m/z calculated for [M H] [C28H6BClF15O] , 687.9997; found, radiation and a CCD detector. Measurements were typically made at 687.9984. 150(1) K with temperatures maintained using an Oxford cryostream. Compound 3e. Yield: 46 mg, 60.8 μmol, 61%. Mp: 58−63 °C. 1H Data were collected and integrated and data corrected for absorption − NMR (500 MHz, CDCl3, 298 K): 8.04 (s, br, 1H, C H), 7.81 (m, 1H, using a numerical absorption correction based on Gaussian integration C−H), 7.47 (m, 2H, Ar−H), 7.36 (m, 2H, Ar−H), 7.16 (s, 1H, C− over a multifaceted crystal model within CrysAlisPro.28 The structures 13 fi 2 H), 2.60 (s, 2H, CH2). C NMR (126 MHz, CDCl3, 298 K): 176.1 (s, were solved by direct methods and re ned against F within SHELXL- br), 166.5 (s, br), 161.9 (s, br), 149.0 (m), 147.0 (m), 146.2 (m), 2013.29 For compounds 5, intensity data sets were collected at low 143.3 (m), 140.9 (m), 138.4 (m), 136.2 (m), 134.40 (s), 133.0 (s), temperature with a Bruker AXS Smart 1000 CCD diffractometer using 132.60 (s), 132.3 (s), 132.11 (s), 131.7 (s), 130.6 (s), 129.8 (s), 127.7 a sealed X-ray tube and graphite monochromator. Data sets for 5 were (s, br), 126.6 (s), 120.5 (s), 117.1 (s, br), 116.2 (m), 27.3 (m), 21.4 corrected for air and detector absorption and Lorentz and polarization

4135 DOI: 10.1021/acs.organomet.5b00546 Organometallics 2015, 34, 4127−4137 Organometallics Article effects;30 absorption by the crystal was treated with a semiempirical Fröhlich, R.; Erker, G. Angew. Chem., Int. Ed. 2008, 47, 7543−7546. multiscan method.31,32 The structures were solved by direct methods (d) Wang, H. D.; Fröhlich, R.; Kehr, G.; Erker, G. Chem. Commun. with dual-space recycling33 (compound 5a) or by intrinsic phasing34 2008, 5966−5968. (e) Geier, S. J.; Chase, P. A.; Stephan, D. W. Chem. (compound 5b) and refined by full-matrix least-squares methods based Commun. 2010, 46, 4884−4886. (f) Greb, L.; Oña-Burgos, P.; on F2 against all unique reflections.35 The crystals of 5a were Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. Angew. Chem., pseudomerohedrally twinned (twin fractions 0.61:0.39); refinement Int. Ed. 2012, 51, 10164−10168. (g) Wang, Y.; Chen, W.; Lu, Z.; Li, Z. was against all reflections involving both twin domains. All non- H.; Wang, H. Angew. Chem., Int. Ed. 2013, 52, 7496−7499. (h) Greb, hydrogen atoms were given anisotropic displacement parameters. The L.; Daniliuc, C. G.; Bergander, K.; Paradies, J. Angew. Chem., Int. Ed. positions of most hydrogen atoms (except those of the methyl groups) 2013, 52, 5876−5879. (i) Hounjet, L. J.; Bannwarth, C.; Garon, C. N.; were taken from difference Fourier syntheses and refined. A summary Caputo, C. B.; Grimme, S.; Stephan, D. W. Angew. Chem., Int. Ed. of crystallographic data are available as Supporting Information, and 2013, 52, 7492−7495. (j) Chernichenko, K.; Madarasz,́ A.; Papai,́ I.; the structures have been deposited with the Cambridge Structural Nieger, M.; Leskela,̈ M.; Repo, T. Nat. Chem. 2013, 5, 718−723. − Database (CCDC deposition numbers 1062326 1062327 and (k) Mahdi, T.; Stephan, D. W. J. Am. Chem. Soc. 2014, 136, 15809− − 1062523 1062531). 15812. Computational Studies. DFT calculations were carried out using (4) (a) Chen, D.; Klankermayer, J. Chem. Commun. 2008, 2130− Jaguar v.8.7.36 and visualized within Maestro 10.1. All structures were * 2131. (b) Chen, D.; Wang, Y.; Klankermayer, J. Angew. Chem., Int. Ed. geometry optimized at the B3LYP/6-31G + level followed by 2010, 49, 9475−9478. (c) Sumerin, V.; Chernichenko, K.; Nieger, M.; subsequent thermodynamic single-point energy calculations at the ̈ − ζ * Leskela, M.; Rieger, B.; Repo, T. Adv. Synth. Catal. 2011, 353, 2093 triple- B3LYP/6-311G + level and a zero point energy correction 2110. (d) Liu, Y.; Du, H. J. Am. Chem. Soc. 2013, 135, 6810−6813. made. Determination of partial charges and bond orders were made − 37 (e) Wei, S.; Du, H. J. Am. Chem. Soc. 2014, 136, 12261 12264. (f) Shi, using the NBO approach. L.; Zhou, Y.-G. ChemCatChem 2015, 7,54−56. (5) (a) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem., ■ ASSOCIATED CONTENT Int. Ed. 2007, 46, 4968−4971. (b) Dureen, M. A.; Stephan, D. W. J. *S Supporting Information Am. Chem. Soc. 2009, 131, 8396−8397. (c) Voss, T.; Chen, C.; Kehr, This material is available free of charge on the ACS publication G.; Nauha, E.; Erker, G.; Stephan, D. W. Chem. - Eur. J. 2010, 16, − Web site: The Supporting Information is available free of 3005 3008. (d) Chen, C.; Fröhlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2010, 46, 3580−3582. (e) Dureen, M. A.; Brown, C. C.; charge on the ACS Publications website at DOI: 10.1021/ − acs.organomet.5b00546. Stephan, D. W. Organometallics 2010, 29, 6422 6432. (f) Liedtke, R.; Fröhlich, R.; Kehr, G.; Erker, G. Organometallics 2011, 30, 5222−5232. Crystallographic data for 2a−c,f,j,k, 3a,b, 5a,b, and 6 (g) Voss, T.; Mahdi, T.; Otten, E.; Fröhlich, R.; Kehr, G.; Stephan, D. (ZIP) W.; Erker, G. Organometallics 2012, 31, 2367−2378. (h) Melen, R. L. NMR spectra, computational data, and a summary of Chem. Commun. 2014, 50, 1161−1174 and references therein. crystallographic data (PDF) (6) Ullrich, M.; Seto, K. S.-H.; Lough, A. J.; Stephan, D. W. Chem. Commun. 2009, 2335−2337. (7) (a) Mömming, C. M.; Kehr, G.; Wibbeling, B.; Fröhlich, R.; ■ AUTHOR INFORMATION Schirmer, B.; Grimme, S.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, Corresponding Authors 2414−2417. (b) Feldhaus, P.; Schirmer, B.; Wibbeling, B.; Daniliuc, C. *E-mail for R.L.M.: MelenR@cardiff.ac.uk. G.; Frohlich, R.; Grimme, S.; Kehr, G.; Erker, G. Dalton Trans. 2012, − *E-mail for M.M.H.: [email protected]. 41, 9135 9142. (c) Feldhaus, P.; Wibbeling, B.; Fröhlich, R.; Daniliuc, C. G.; Kehr, G.; Erker, G. Synlett 2014, 25, 1529−1533. Notes fi (8) Melen, R. L.; Hansmann, M. M.; Lough, A. J.; Hashmi, A. S. K.; The authors declare no competing nancial interest. Stephan, D. W. Chem. - Eur. J. 2013, 19, 11928−11938. (9) (a) Hansmann, M. M.; Melen, R. L.; Rominger, F.; Hashmi, A. S. ■ ACKNOWLEDGMENTS K.; Stephan, D. W. J. Am. Chem. Soc. 2014, 136, 777−782. For further M.M.H. is grateful to the Fonds der Chemischen Industrie for a reactions of B(C6F5)3 with propargyl esters see: (b) Hansmann, M. M.; Melen, R. L.; Rominger, F.; Hashmi, A. S. K.; Stephan, D. W. Chemiefonds scholarship and the Studienstiftung des deut- − schen Volkes. D.W.S. is grateful for the award of a Canada Chem. Commun. 2014, 50, 7243 7245. (10) Brooner, R. E. M.; Widenhoefer, R. A. Angew. Chem., Int. Ed. Research Chair. We thank W. R. Grace for the kind donation of 2013, 52, 11714−11724. B(C6F5)3. (11) Zhao, X.; Stephan, D. W. J. Am. Chem. Soc. 2011, 133, 12448− 12450. ■ REFERENCES (12) Stirling, A.; Hamza, A.; Rokob, T. A.; Papai,́ I. Chem. Commun. (1) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. 2008, 3148−3150. Science 2006, 314, 1124−1126. (13) Guo, Y.; Li, S. Eur. J. Inorg. Chem. 2008, 2008, 2501−2505. (2) For recent reviews on FLP reactivity and activation of small (14) Kehr, G.; Erker, G. Chem. 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4137 DOI: 10.1021/acs.organomet.5b00546 Organometallics 2015, 34, 4127−4137